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PREPARATION FOR LABORATORY SAMPLES - Coggle Diagram
PREPARATION FOR LABORATORY SAMPLES
Making Samples Homogeneous
Grinding
Important for sample preparation prior to analysis and for food ingredient processing
available for reducing particle in size to achieve sample homogenization
some foods are more easily ground after drying in a desiccator or vacuum oven
Grinding wet sample may cause significant losses of moisture and chemical changes. In contrast, grinding frozen samples reduces undesirable changes.
The grinding process should not heat the sample
The grinder should not be overloaded because heat will be produced through friction
For heat-sensitive sample, grinders can be cooled with liquid nitrogen and the ground samples are stored at -80 degree celcius
Contact of food with bare metal surfaces should be avoided if trace metal analysis is to be performed
Another altervative is
cryogenic grinding or cryogrinding
Cryogenic grinding is ideal for biological samples and materials that are sensitive to oxygen or temperature. Most materials are suitable for this technique
Cryogrinding can be performed manually with a mortar and the pestle after freezing, the sample with liquid nitrogen
The mortar and pestle have to be prechilled with liquid nitrogen before adding the material
There are several brands of specialized grinding equipment with an integrated cooling system that perform the cryogenic freezing and grinding automaticaly
Reducing Sample Size
If the particle size or mass of the sample is too large for analysis, it must be reduced in bulk or particle size using quartering technique
To obtain a smaller quantity for analysis, the sample can be spread on a clean surface and divided into quarters. The two opposite quarters are combined.
If the mass is still too large for analysis, the process is repeated until an appropriate amount is obtained. This method can be modified for homogeneous liquid by pouring into four containers and can be automated.
The sample are thus homogenized to ensure negligable differences between each portion.
Once the sample has been homogenous, a small portion is selected for analysis.
Determination of Particle size
Particle size is controlled in certain mills by adjusting the distance between burrs or blades or by screen mesh size/number
The mesh number is the number of square screen openings per linear inch of mesh
The final particles of dried foods should be 20 mesh for moisture, total protein, or mineral determinations.
Particles of 40 mesh size are used for extraction assays such as lipid and carbohydrate estimation.
To reducing particle size for analysis of sample, it also important to reduce the particle size of many food ingredients for use in specific food products.
The simplest way to measure particle sizes of dry materials of less than 50 nanometer in diameter is by passing the sample through a series of vertically stacked sieves with increasing mesh number
As the mesh number increase, the apertures between the mesh are smaller and only finer and finer particles pass through subsequent sieves
Sieves size have been specified for salt, sugar, wheat flour, cornmeal, semolina, and cocoa
The sieve method is not suitable for emulsions or very fine powders.
Sample Identification
The sample should include
Sample description
time samples taken
location sample was taken from
person who took the sample
method used to select the sample
Preventing Changes in Sample
Enzymatic Inactivation
Food materials often contain endogeneous enzymes that may degrade the food components being analyzed
Enzyme activity therefore must be eliminate or controlled using methods that depend on the nature of the food.
Heat denaturation to inactivate enzymes and freezer storage (-20 to -30C ) for limiting enzyme activity are common methods.
Some enzymes are more effectively controlled by changing the pH or salting out
Oxidative enzymes may be controlled by adding reducing agents.
Lipid Protection
Lipids present particular problems in sample preparation
High-fat foods are difficult to grind and may need to be ground while frozen
Unsaturated lipids are sensitive to oxidative degradation and should be protected by storing under nitrogen or vacuum
Antioxidants may stabilize lipids and may be used if they do not interfere with the analysis
Light-initated photooxidation of unsaturated lipids can be avoided by controlling storage conditions.
In practice, lipids are more stable when frozen in intact tissue rather than as extracts
Ideally, unsaturated lipids should be extracted just prior to analysis
Low temperature storage is generally recommended to protect most foods.
Microbial Growth and Contamination
Microorganisms are present almost all foods and can alter the sample composition
Microorganisms are present on all but sterilized surfaces, so sample cross-contamination can occur if samples are not handled carefully
Freezing, drying, and chemical preservatives are effective control
The preservation methods used are determined by the probability of contamination, the storage conditions, storage time, and the analysis to be performed
Physical changes